PURPOSE: To attempt to reduce noise as well as to enhance the cooing efficiency by arranging that the shape of a sound is properly changed in conformity with the difference in the condition of a draught system at the time of high pressure loss or low pressure loss of an axial flow fan.

CONSTITUTION: At the idling time of a high draught system, a working ring 20 is placed at the position a little to the front, and the boundary line 2c of an air conducting part 17 and a releasing part 16 is placed at the adjacent position of the front edge 4a of an oil flow fan 4 to make the forward open angle θ of the releasing part 16 large. On that account, the inflow air along the release part 16 from the blade tip of the fan 4 to the conducting part 17 is decreased, and the turbulence generated at the blade tip of the fan is removed by the leading-edge of the conducting part 17 and the noise generation is reduced. As the loss in the air blasting is decreased when the turbulence and the centrifugal flow at the conducting part 17 are further removed, the cooling efficiency of a radiator 1 is enhanced. Next, since the ring 20 is placed a little to the center when changed to the time of low draught system, and the position of the boundary line 2c is separated from the fan 4 to the extent of distance L making the angle θ small, the turbulence at the blade tip of the fan 4 is made small and the reduction of noise generation and enhancement of cooling efficiency become possible.